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THE COLORADO RIVER PROBLEM 317
not more than 1,00 ft. wide for about 9 miles. This section of the river has never given serious flood trouble. The upper end of the old delta begins at this point, but in its present condition the river does not take the character-of a stream in its delta until it reaches the Pescadero Cut-off about 30 miles below. In the stretch from 9 to 40 miles below Yuma, the river now meanders with about the same characteristics as it does above Yuma. Just below the Pescadero Cut-off, the river spreads on its present delta cone. The area covered by the spread is relatively small and the accretions are corre-spondingly rapid. As the cone builds up, the delta condition probably will extend up stream so that the banks will eventually overflow at high water for some distance above the Pescadero Cut-off no matter how much the maximum flood is reduced. The normal slope of the Colorado is 1.1 ft. per mile. Under present conditions, when the Yuma gauge is below 25.4 ft., the river does not ordinarily get over its banks on the Yuma Project from Laguna to Mile 16 on the levee below Yuma, nor on the Imperial side above the junction of the Ockerson and O. D. Co. Levees. The river has passed more than 120,000 sec-ft. at this gauge height and the curves on Fig. 5 show that if the rate of increase in flow is properly retarded, the river will pass at least 80,000 sec-ft. at this stage. Below the 16-mile point, the banks are not so high above low water and the river shows a disposition to widen and not cut so deep at high water, so that the banks near the Pescadero Cut-off now overflow at a discharge of 35,000 to 50,000 sec-ft. (See Fig. 6.)
[Fig. 5. COLORDO RIVER - CURVE SHOWING RELATION OF GAUGE HEIGHTS AND DEPTH OF SCOUT TO DISCHARGE AT YUMA]
Scouring action varies theoretically as the square of the velocity. On the Colorado, active scour begins at a velocity of about 4.5 ft. per sec. and a dis-charge of 10,000 sec-ft. The velocity increases with discharge until a flow of about 26,000 sec-ft. is reached, at which discharge the average velocity is 6.2